As performance required for an air cleaner, there are fundamental performance such as low noise, electric power saving, high dust collection, and high deodorization. In addition, in recent years, washing and reuse of a filter unit have been required.
Previously, many of filter portions set in the air cleaner have been arranged so that they are separated into a dust collecting portion and a deodorizing portion. Currently, many of dust collecting portions mainly used in a market are HEPA filtering medium, and a deodorizing portion is plate-like in many cases. Since the dust collecting portion is a filtering medium of HEPA (a ultrahigh performance filtering medium which can collect 99.97% or more 0.3μ particles) performance having a high airflow resistance, the portion is arranged separately from the deodorizing portion. In addition, in order to reduce a structure pressure drop as a filter unit and, at the same time, perform large air volume treatment in conformity with treatment ability at the deodorizing portion, mini-pleats processing at 4 millimeter pitch or less is required to increase an area of a filtering medium to be used. For this reason, the dust collecting portion and the deodorizing portion are arranged in such a separated shape.
However, for washing and reuse of the filter portion which have been required in recent years, a filter having such a structure is necessary to be washed two times, one time for the dust collecting portion and the other time for the deodorizing portion, and integration of the dust collecting portion and the deodorizing portion is required for simplicity. This integration is effective in not only simplicity of washing, but also ease of filter exchange and cost saving. However, since simple addition of the deodorizing function to the dust collecting portion increases a thickness of the filtering medium, an amount of the filtering medium which can be accommodated in the unit is decreased, and a unit pressure drop due to rise in a structure pressure drop is increased. Decrease of filtering medium and increase of unit pressure drop occur problems such as reduction in a treatment air volume, increase in a noise, increase in consumed electric power, and reduction in a collecting efficiency, therefore integration is extremely difficult.
For integration, Patent Literature 1 discloses an air cleaning filter element obtained by immersing a woven fabric, a knitted fabric or a nonwoven fabric in a latex liquid containing an adsorbent, drying the fabric after attachment of the adsorbent to obtain a sheet with the adsorbent attached thereto, and subjecting a filtering medium to which an electret nonwoven fabric is adhered to pleats processing. However, in an aspect specifically described in Example of this literature, a substrate to which the adsorbent is attached is a knitted fabric or a urethane foam, a thickness is as thick as 1 mm or more to 3 mm. A thick filtering medium like this does not suit a filtering medium which is subjected to mini-pleats processing for large air volume treatment.
In addition, Patent Literature 2 discloses a deodorizing electret filter in which an electret nonwoven fabric is laminated on a deodorizing filter obtained by supporting a deodorizing agent on an air-permeable substrate through coating or immersion. A dry nonwoven fabric and a wet type non-woven material are disclosed as examples of a nonwoven fabric of the air-permeable substrate. As a specific nonwoven fabric, a resin bond (chemical bond) nonwoven fabric obtained by fixing fibers to a dry nonwoven fabric consisting of a polyacrylate fiber and a polyvinyl chloride fiber using vinylidene chloride, is merely disclosed. However, since the dry nonwoven fabric has a high weight variation rate, a deodorization rate is easily reduced. In addition, in the case of a crimped fiber, since loosing property due to crimping necessarily remains, even when fixed with a resin, the fabric is easily stretched, and becomes extremely soft, being not suitable for pleats processing.
Next, Patent Literature 3 discloses a deodorizing sheet in which a porous material having a specified particle diameter range (e.g. 60 to 600 μm) is wet-adhered and fixed on a nonwoven fabric, as a structure of a filtering medium. In the deodorizing sheet produced by such a method, big particles of a porous material are held between fibers of a nonwoven fabric, therefore, a thickness of the filtering medium becomes great, and an area of the filtering medium which can be accommodated in the filter unit is decreased. Further, when the sheet is pleats-processed, since a pressure drop at a folded back part of the filtering medium easily becomes great, it is difficult to reduce a pressure drop. In addition, since the porous material is wet-adhered and fixed, even if small particles are used, particles are densely deposited on particles and a space required for airflow becomes narrow, therefore, a pressure drop is easily increased.
Like this, it cannot be said that all of the previous techniques for integrating dust collection and deodorization is effective technique relating to manufacturing of a filtering medium suitable for mini-pleats processing.
On the other hand, in order to perform large air volume treatment with a mini-pleats-type filter at a low pressure drop, it is important to reduce a pressure drop of the filtering medium itself, but it is also important to reduce a structure pressure drop, because the pressure drop of the filter unit is a sum of the pressure drop of the filtering medium itself, and the structure pressure drop generated when the filtering medium is pleats-processed and accommodated. For this reason, a hard filtering medium which is thin, and is not deformed with an air pressure is necessary.
Patent Literature 4 shows trial of suppressing generation of a structure pressure drop by thinning and hardening a filtering medium, and discloses application of a wet type nonwoven material including modified cross-section yarns of a rayon fibers or glass fibers, as a fiber constituting a support of the filtering medium. However, the filtering medium including thick glass fibers have a problem on safety and hygiene because the glass fibers are broken and flown at a pleats processing step. In addition, since when a support having a high fiber density is coated with a gas adsorbing substance, clogging occurs frequently, the pressure drop of the filtering medium becomes high.
Further, Patent Literatures 5 and 6 disclose that a filtering medium is composed of a fiber having a high Young's modulus to enhance dimensional stability and a strength for an air pressure.
Among them, Patent Literature 5 relates to a filtering medium for a bag filter, and heat resistant fibers such as polyphenylene sulfite fibers having a Young's modulus of 20 cN/dtex or more. However, since this filtering medium is constructed of a nonwoven fabric or a woven fabric obtained by entangling crimped fibers with needle punching into a high-bulky form, and the fabric is not fixed between the said fibers, the medium is deformed with an air pressure, and a structure pressure drop is easily caused.
In addition, Patent Literature 6 discloses a process for producing a flame-retardant filtering medium using polyvinyl alcohol fibers and polyester fibers by a paper making method, and it is described that, as a feature of the paper making method, fibers having different thicknesses and lengths can be used, and fibers having a high Young's modulus can be also used. However, in this literature, it is only described that, when the paper making method is used, fibers having a high Young's modulus can be used and a specific numerical value regarding a Young's modulus of a fiber is not described at all. For this reason, there is a possibility that a filtering medium in which, when a gas adsorbing substance is adhered, clogging occurs, and air permeability is remarkably reduced in some cases, is obtained.
Based on the above reasons, previously, a filter unit in which both functions of dust collection and deodorization can be performed by one filtering medium, and which can be subjected to large air volume treatment at a low pressure drop, was not present.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 4-74505    Patent Literature 2: JP-A No. 2000-279505    Patent Literature 3: JP-A No. 2000-117024    Patent Literature 4: JP-A No. 2002-1020    Patent Literature 5: International Publication WO 04/87293    Patent Literature 6: JP-A No. 2006-136809